166 AQUATIC PHYCOMYCETES 



loosely in the swollen, sometimes walled-off, portions of the host, 

 upon germination functioning as a sporangium. 



The taxonomic status of Pleolpidium and Rozella has been discussed 

 in detail (Sparrow, 1938b; Karling, 1942b). 



The observations of Foust (1937) on Rozella allomycis indicate that 

 in some (but not all) specimens a single thallus may not only com- 

 pletely fill an infected part, but may also sometimes segment and, 

 as in R. septigena, form several sporangia. If this segmentation is clearly 

 shown not to be due to multiple infection then the diagnosis above 

 should be modified to include this sorus formation, or a new genus 

 should be erected for R. septigena Cornu, R. allomycis, and R. aehlyae. 

 Germination of the spiny resting spores has been secured by Foust 

 and reinfection by means of the zoospores has been observed. The 

 latter spores formed the typical sporangial stage, proving unquestion- 

 ably that the two phases are related. Resting-spore germination by 

 means of zoospores was observed by Karling (1942a), in R. cladochytrii. 

 Butler (1907) concluded from observations on a number of species of 

 Rozella that in some instances at least the plasma from several spores 

 may unite in the host to form a true Plasmodium, from which a single 

 sporangium may arise. He does not, however, exclude the possibility 

 that one thallus has gained the ascendency and developed at the ex- 

 pense of the rest. 



The species are all parasitic in the hyphae and reproductive organs 

 of other aquatic Phycomycetes. Although customary to consider that 

 each species is restricted to a single host genus (or even species), in 

 most instances the truth of this supposition awaits confirmation. Ob- 

 servations on a Rozella parasitic in Dictyuchus anomalus by Johnson 

 (1955a) point to an even wider host range than suspected. Johnson was 

 able to transfer his fungus not only to D. monosporus but to several 

 species of Achlya as well. From this he concluded his fungus was Ro- 

 zella aehlyae and he suggests that past failures to induce artificial in- 

 fection with species of Rozella (in other than the original host) may 

 have been due to an inherent difference with respect to susceptibility 

 and resistance. Even certain strains of D. anomalus appeared resistant 

 to his material of R. aehlyae. Furthermore, he believes that biological 

 races of the parasite may exist which differ in their pathogenicity. 



